One of the characteristics of living cells is that they constantly synthesize and degrade protein. The cell expands considerable energy on these processes and, seemingly, derives no beneficial net result. That is, qualitatively and quantitatively protein content remains relatively invariable (under "normal" conditions). Under such circumstances it seems reasonable to ask- why have cells involved themselves in such a process if the gain, as we presently see it, seems, at best, meager compared to the cost? Our speculation is that by having both protein synthesis and degradation going on at all times, the cell, via appropriate control devices, can rapidly increase or decrease the rates of these processes so that the concentration of a given protein can be altered expeditiously. Thus, our present concept is that the capacity to specifically and rapidly regulate enzyme activity is the price that is paid for the apparently wasteful process commonly referred to as protein turnover. Now, how does the cell operate a system responsible for the individual regulation of hundreds of proteins? With respect to mammalian cells we are essentially devoid of any substantial information in this area.